The world contains not only causes and effects, but also causal relations holding between causes and effects. Because causal relations enter into the structure of the world, their presence has various modal and probabilistic consequences. Causation and “necessary and sufficient conditions” do often go hand in hand. Causation, however, is a robust ingredient within the world itself, whereas modalities and probabilities supervene on the nature of the world as a whole, and on the resulting relations between one possible world and others. Some modalities, therefore, are essentially causal; but causation is not essentially modal.19